The present invention relates to an electron gun system suitable for use in an electron microscope, an electron beam lithography or the like, and more particularly to the improvement on a field emission electron gun system suitable for acquisition of a high-brightness and large-current electron beam.
In the conventional field emission electron gun system, approaches of acquiring a large-current electron beam by use of not only an electronic beam focusing action with an electrostatic lens but also a magnetic beam focusing action with a magnetic lens disposed in the vicinity of an electron source have been disclosed by British Patent No. 1,291,221 and JP-A-59-42748.
In order to obtain a desired focused electron beam in using an electron gun in a transmission or scanning electron microscope, an electron beam lithography or the like, there is usually employed an illuminating system, that is, a system in which the electron gun is combined with a condenser lens(es). However, the luminance level (or brightness) of the electron beam obtained becomes lower than that of an electron source itself due to aberrations of the electron gun and the condenser lens. In order to obtain a high-brightness electron beam, it is important to reduce the aberrations of the illuminating system (mainly including spherical aberration and chromatic aberration) as greatly as possible, thereby maintaining the luminance level intrinsically possessed by the electron gun.
Electrons undergo an electrostatic lens action when they are accelerated in an electron gun. In the conventional illuminating system in electron microscopes or the like, there cannot exist together a condition that the aberration of the electrostatic lens in the electron gun is small and a condition that the aberration of the condenser lens is small. The spherical aberration and chromatic aberration of the electrostatic lens in the electron gun is about ten times as great as those of a magnetic lens used in an electron microscope or the like. Therefore, a magnetic lens is provided in the vicinity of the electron source for the purposes of reducing the aberration of the electron gun and making up for the electrostatic lens action and the electrostatic lens action is controlled to allow the system to operate in a condition in which the aberration of the condenser lens becomes small. As a result, a high-brightness and large-current electron beam is obtained.
The electron gun systems according to the prior art mentioned in the above employ a single-stage acceleration tube used when an acceleration voltage is low. Therefore, an electron accelerating region is short in distance and an electrostatic lens action is weak. Further, since the distance between the condenser lens and the electron source is also short, the aberration caused by reducing the beam size by the condenser lens is also small. Accordingly, no substantial problem has arised in the conventional electron gun systems with the single-stage acceleration tube used under a low acceleration voltage condition. However, a magnetic field superposed field emission electron gun system with a multi-stage acceleration tube used under a high acceleration voltage condition has involves problem. Namely, when a real image of a field emission electron source is formed in a direction of progression of an electron beam by a magnetic lens, as disclosed by the JP-A-59-42748, a strong electrostatic lens action makes large the angle of incidence of the electron beam upon a condenser lens. Also, in the magnetic field superposed field emission electron gun system using the multi-stage acceleration tube, since an electron accelerating region is long, the distance from the field emission electron source to the condenser lens is long and hence the greater degree of reduction of the beam size by the condenser lens is required for allowing the formation of a beam spot by the condenser lens. This results in the deterioration of brightness due to the spherical aberration of the condenser lens. Thus, a condition in the magnetic field superposed field emission electron gun system with the multi-stage acceleration tube for acquiring a high-brightness and large-current electron beam by the illuminating system in the combination of the electron gun with the condenser lens is different from that in the field emission electron gun with the single-stage acceleration tube. Further, in the electron gun system according to the prior art mentioned above, no consideration is made with a view to keeping the illuminating conditions such as the electron beam spot position and the beam divergence angle unchanged when the operating conditions of the electron gun such as the field emission voltage and the acceleration voltage are changed. Also, in the case of the magnetic field superposed field emission electron gun system using the multi-stage acceleration tube, since a high voltage is applied, there is a large danger that a discharge may occur. Therefore, it is necessary to provide a power source construction which is not liable to undergo any damage even upon occurrence of the discharge at the electron gun or the high-voltage sources. However, when the conventional construction is employed for a power source for applying a field emission voltage, a power source for applying a voltage to a first-stage acceleration electrode and a power source for applying voltages to second-stage to final-stage acceleration electrodes, there is a problem that it is hard to take a protective measure to counter the discharge since there becomes large a possibility that a discharge current flows between a plurality of power sources when the output of each power source is short-circuited by the discharge.